I need to sort out something about the IPv4 header. For example the following frame with an Ethernet-II frame with an IPv4 packet starting at the fifteenth byte.
0000: 08 00 20 7c 94 1c 00 00 - 39 51 90 37 08 00 45 00
0010: 00 3e 36 00 00 00 80 11 - da 4f 82 eb 12 7f 82 eb
0020: 12 0a 04 01 00 35 00 2a - ee 6a 00 01 01 00 00 01
0030: 00 00 00 00 00 00 06 67 - 65 6d 69 6e 69 03 6c 64
0040: 63 02 6c 75 02 73 65 00 - 00 01 00 01
I need to sort somethings out:
What does the 0000 & 0010 & 0020 & 0030 on the left stands for?
I just cant sort it out is 1 pair for example the first one 08 two bits or?
And if the IPv4 starts at fifteenth byte(1 byte = 8 bits) where does it start then, have problems to understand this because i dont get number 2.
Thank you for your time.
”45” in your first line of hexdump is the 1st byte of the ip header (15th byte of the ethernet frame). Each line is 16 bytes.
Also, in the beginning of each line has an offset like e.g. ”0010: ” (in hex) means the starting offset from the start of the whole dump.
Your first line would be, (total 16 bytes),
dmac(6)+smac(6)+etype(2)+ first2byte_of_ip(2)
and your first byte of ip is hex ”45”, you can lookup the detail ip header field in wikipedia.
It would be nice if you can read wireshark user's guide on your own. Anyway, to answer your question,
1) What does the 0000 & 0010 & 0020 & 0030 on the left stands for?
It stands for hexdump offset. You can refer to this page.
2) I just cant sort it out is 1 pair for example the first one 08 two bits or?
It is (part of ) destination MAC address. Entire MAC address should be 08 00 20 7c 94 1c.
3) since Q2 is now answered, this should not be problem for you.
Related
It's currently 04:40 AM and I am stuck on something I simply do not understand. I am trying to look up a domain's nameservers directly by using the DNS protocol. If I send a host -t ns google.com 1.1.1.1 and monitor it with Wireshark, I can see the full query of the DNS query. However, I cannot figure out, why some ASCII characters are used one time, but not another time. Here is an example:
0000 70 4d 7b 94 dd e0 00 d8 61 a9 c5 ec 08 00 45 00 pM{.....a.....E.
0010 00 38 d6 ff 00 00 80 11 9f 50 c0 a8 01 bb 01 01 .8.......P......
0020 01 01 e8 40 00 35 00 24 a0 19 9e f7 01 00 00 01 ...#.5.$........
0030 00 00 00 00 00 00 06 67 6f 6f 67 6c 65 03 63 6f .......google.co
0040 6d 00 00 02 00 01 m.....
In this DNS query, I am looking up the nameservers for google.com. The actual query starts at 06 07.
06 in ASCII is ACK/Acknowledgment.
Now, if we take a look at gmail.com instead:
0000 70 4d 7b 94 dd e0 00 d8 61 a9 c5 ec 08 00 45 00 pM{.....a.....E.
0010 00 37 d7 00 00 00 80 11 9f 50 c0 a8 01 bb 01 01 .7.......P......
0020 01 01 e8 58 00 35 00 23 8f cc 6f e2 01 00 00 01 ...X.5.#..o.....
0030 00 00 00 00 00 00 05 67 6d 61 69 6c 03 63 6f 6d .......gmail.com
0040 00 00 02 00 01 .....
the query starts at 05 67 instead.
05 is ENQ/Enquiry.
Why are they different? If I try to send 06 instead of 05 the DNS server gives me no response but Wireshark tells me:
Unknown extended label
I've seen 05, 06, and 09 so far. 09 is my biggest "wat" of all time, because it's a HT/Horizontal Tab.
Anyone with a lot of DNS knowledge who can help me here? I'm not looking for "just use dig/nslookup/host command". I'm currently trying to research a bit on the DNS protocol, and this is a thing I do not understand.
Good read where I got a lot of help: http://dev.lab427.net/dns-query-wth-netcat.html
For a binary protocols like this, you can't assume each byte corresponds to the matching ASCII character.
Take a look at section 4.1.2 of the DNS RFC (https://www.ietf.org/rfc/rfc1035.txt).
The domain name in a DNS request is broken up into "labels". For each label, the first byte is the length of the label, then the bytes for the string are written.
For your Google.com example, the labels are "google" and "com". The 06 is the number of bytes in the first label. This is followed by the bytes for "google". Then the 03 is the number of bytes in the "com" label. After the "com" bytes, the 00 byte is the NULL label to mark the end.
I have a packet that I have manually created for a SYN/ACK but I get no reply from the server.
This is all wireless/GSM stuff so I cannot use a sniffer.
I have calculated the TCP and the IP header checksums manually a few times and they seem correct but I really need a 3rd party method to be sure.
I had several endian issues but I think I have it right now. But who knows...
I only found an online parser but it does not test/verify the checksums.
Does anyone have an easy idea for me?
Just in case someone has suitable access to a test method, and feels like pasting it in for me, here is the packet:
45 10 00 3C 00 02 00 00 64 06 E8 1F 0A AA 61 43 51 8A B1 13
01 BB 01 BB 00 00 00 0A 00 00 00 00 50 02 00 00 3D D8 00 00
Regards
berntd
I've creating a pcap from your hex data using Net::PcapWriter:
use strict;
use warnings;
use Net::PcapWriter;
my $w = Net::PcapWriter->new('test.pcap');
my $ip = pack('H*','4510003C000200006406E81F0AAA6143518AB11301BB01BB0000000A00000000500200003DD80000');
$w->packet($w->layer2prefix('1.1.1.1').$ip);
Loading it into Wireshark shows both the IP checksum and the TCP checksum as correct, so it is probably not a problem of the checksum calculation.
But tcpdump says that the length is wrong:
IP truncated-ip - 20 bytes missing! 10.170.97.67.443 > 81.138.177.19.443: Flags [S], seq 10:30, win 0, length 20
This is because you've set the total length in the IP header to 60 bytes (00 3C) but the IP header + TCP header is only 40 bytes in total and your packet does not have any payload, i.e. the total length should be 40 and not 60 bytes.
Here is what I came up with to do it the manual way:
Put packet into a text file like so:
45 10 00 3C 00 02 00 00 64 06 E8 1F 0A AA 61 43 51 8A B1 13
01 BB 01 BB 00 00 00 0A 00 00 00 00 50 02 00 00 3D D8 00 00
add addressing offsets and group into 16 byte lines as in a hex dump:
000000 45 10 00 3C 00 02 00 00 64 06 E8 1F 0A AA 61 43
000010 51 8A B1 13 01 BB 01 BB 00 00 00 0A 00 00 00 00
000020 50 02 00 00 3D D8 00 00
Save it (source).
Now run ext2pcap.exe -e 0x800 source dest
The dest file can now be imported as a PCAP file into wireshark for decoding.
Multiple packets can be converted byt starting the address offset for each new packet at 000000 again in the source file.
text2pcap.exe seems to come with wireshark.
Tedious but works.
Cheers
I am trying to calculate the checksum of a tcp packet and I can't get the same value as in the packet captured with wireshark
the original captured packet is:
"6c f0 49 e8 a3 0d 24 b6 fd 52 40 cb 08 00 45 00 00 28 02 22 40 00 80 06 00 00 00 0a 2a 00 1c 1f 0d 5a 24 ca 7d 01 bb 3f 44 f8 6e 6c 83 75 20 50 10 01 02 83 91 00 00"
As I saw in wireshark:
The first 14 bytes are ETH.
After that (in the IP part) there are 12 bytes of "header length",'DSCP','total length,'identification','fragment offset','TTL','protocol','header checksum'.
and then there are 4 bytes of IP-src and 4 bytes of IP-dst (which are the only one in the IP header that are important for the calculation).
we are left with 20 bytes of TCP header (no data).
I created the new packet for the calculation with the pseudo header in the form:
IPsrc/IPdst/reserved(0x00)/protocol(0x06)/TCP-length(0x0014)/TCP-header
Which got me:
"0a 2a 00 1c 1f 0d 5a 24 00 06 00 14 ca 7d 01 bb 3f 44 f8 6e 6c 83 75 20 50 10 01 02 83 91 00 00"
Zeroing the tcp checksum field (the 0x8391 according to wireshark) gets:
"0a 2a 00 1c 1f 0d 5a 24 00 06 00 14 ca 7d 01 bb 3f 44 f8 6e 6c 83 75 20 50 10 01 02 00 00 00 00"
calculating checksum on the new packet got me the value: 0xcc45 which is differen than the one in the original packet (0x8391)
data="0a 2a 00 1c 1f 0d 5a 24 00 06 00 14 ca 7d 01 bb 3f 44 f8 6e 6c 83 75 20 50 10 01 02 00 00 00 00"
def carry_around_add(a, b):
c = a + b
return (c & 0xffff) + (c >> 16)
def checksum(msg):
s = 0
for i in range(0, len(msg), 2):
w = ord(msg[i]) + (ord(msg[i+1]) << 8)
s = carry_around_add(s, w)
return ~s & 0xffff
data = data.split()
data = map(lambda x: int(x,16), data)
data = struct.pack("%dB" % len(data), *data)
print ' '.join('%02X' % ord(x) for x in data)
print "Checksum: 0x%04x" % checksum(data)
What am I doing wrong?
Your total test data length is 55 bytes, it should be 14 (ETH) + 20 (IP) + 20 (TCP). It's not that it's just one byte too long since that would not match with the 0x8391 checksum. But, the IP Protocol byte (0x06 - TCP) at IP offset 9 is stil in place. The IP source address seems rather unlikely though: 0.10.40.0.
The 'which got me' part starts with '0a 2a' which are the 2nd and 3rd byte of the IP source address. So let's assume the three zero bytes before this address should actually be two zero bytes. If that's correct, we're still on the right track.
The 12 byte TCP pseudo-header is: IPsrc/IPdst/0x00/0x06/TCP-length(0x0014) (without the TCP header), so far so good. The TCP checksum is calculated over: pseudo-header + TCP header + TCP data. But your test data uses only the pseudo-header and the TCP header data, the TCP data itself is missing.
I didn't really check your Python code but I don't see any obvious problems. The main issue seems to be the lack of the TCP payload data in the calculation.
I'm trying to do some reverse engineering on my heating system. Monitoring the CAN BUS results in receiving hexademical strings, for example:
00 D0 68 D6 86 83 61 8F
61 C0 02 5C 12 B5 02 5C
12 78 04 39 04 03 05 02
05 C4 04 5C 12 5C 12 5C
12 5C 12 D0 68 00 00 00
00 18 08 37 D2 00 00 00
00 00 00 00 00 15 75 F2
F0 01 00 01 00 00 00 1F
I know that for example the temperature value of 22.5°C should be somewhere in there.
So far I have tried to look for the following conversions:
Possibility 1: ascii to hex
22.5 = 32 32 2E 35
Possibility 2: float to hex conversion
22.5 = 0x 41 b4 00 00
However none of these resulted in a match.
What would be other possibilities to converted a float to a hex string?
Thx
note: the given string is just a small part of my can sniffer so don't look for 22.5 in my given string here. I'm just looking for other possible conversions.
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Closed 9 years ago.
I've recorded some network traffic in my home that only appear up while running BitTorrent or uTorrent.
I've been reading the bittorrent protocol descriptions, but I am stuck trying to figure out a particular network flow.
Can someone help me identify what the following bittorrent network traffic is exactly?
It lasts quite a long time, even after stopping downloads.
All packets are in one direction - from my local machine running Bittorrent to a remote machine.
Here is data payload of one packet (copied from Wireshark):
00000000 60 00 00 00 00 00 3b 15 20 01 00 00 9d 38 6a b8 `.....;. ....8j.
00000010 04 b9 18 bf 9c 90 d8 81 20 01 00 00 9d 38 6a b8 ........ ....8j.
00000020 20 5a 01 45 bd 13 b1 65 01 04 44 4a e7 d5 04 04 Z.E...e ..DJ....
00000030 01 00 00 00 05 02 ea cf ........
All the packets in the network flow are similar, here are two more:
00000038 60 00 00 00 00 00 3b 15 20 01 00 00 9d 38 6a b8 `.....;. ....8j.
00000048 04 b9 18 bf 9c 90 d8 81 20 01 00 00 9d 38 6a b8 ........ ....8j.
00000058 20 5a 01 45 bd 13 b1 65 01 04 08 8e 35 9f 04 04 Z.E...e ....5...
00000068 01 00 00 00 05 02 ea cf ........
00000070 60 00 00 00 00 00 3b 15 20 01 00 00 9d 38 6a b8 `.....;. ....8j.
00000080 04 b9 18 bf 9c 90 d8 81 20 01 00 00 9d 38 6a b8 ........ ....8j.
00000090 20 5a 01 45 bd 13 b1 65 01 04 12 3e ba 6c 04 04 Z.E...e ...>.l..
000000A0 01 00 00 00 05 02 ea cf ........
These bittorrent packets are typically several seconds apart, and this flow seems to go on indefinitely. Which one of the bittorrent protocols describes this network flow?
I just sent a response to you on our mailing list, but I'm gonna post here too in case anyone else stumbles across it and finds it useful.
They're Teredo packets (with no payload). Wireshark can decode these
but it doesn't do so without coercion.
http://en.wikipedia.org/wiki/IPv6_packet
http://en.wikipedia.org/wiki/Teredo_tunneling
One of your packets dissected:
IP Version: 6
Traffic Class: 0 0
Flow Label: 0 00 00
Payload Length: 00 00
Next Header: 3b (indicates that there is no payload present)
Hop Limit: 15
Source: 20 01 00 00 9d 38 6a b8 04 b9 18 bf 9c 90 d8 81
Destination: 20 01 00 00 9d 38 6a b8 20 5a 01 45 bd 13 b1 65
The source and destination also encode the source and destination
public ipv4 addresses and ports.
The hop-by-hop options header (in type-length-value format) follows in
this case. The possible types can be found here:
http://www.iana.org/assignments/ipv6-parameters/ipv6-parameters.xml
So we have this:
01 04: c3 ae 60 38 ("PadN", random bytes)
04 04: 01 00 00 00 ("Tunnel Encapsulation Limit")
05 02: ea cf ("Router Alert")
No clue what the value of the router alert field is here. I would
expect it to be listed here:
http://www.iana.org/assignments/ipv6-routeralert-values/ipv6-routeralert-values.xml
But it looks like either that's out of date or the Teredo
implementation you're using is doing something non-standard (or
there's something I've missed).
Anyways, these are clearly keep-alive packets. We're not directly
triggering them in the client as far as I know. I believe they're sent
by your Teredo driver to keep your tunnels open.